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Science: Breeding Tomatoes to Look Pretty Sacrifices Their Sweetness

For about 70 years, breeders have selected tomato varieties with uniformly light green fruit before ripening. These tomatoes then turn red evenly as they ripen, and they look nice in a supermarket display.

Researchers now have pinpointed the molecular changes responsible for this “uniform ripening” trait of many modern tomatoes. But these changes, they show, also reduce the fruit’s sugar content.

Ann Powell of the University of California, Davis and colleagues report that the gene at the heart of uniform ripening encodes a protein called GLK2. This protein increases the fruit’s photosynthetic capacity, helping along the production of sugars and lycopene, the pigment that gives a ripe tomato its brilliant color.

Patterns of ripe fruit pigmentation and associated nutrient quality are under the direction of UNIFORM/SlGLK2 transcription factor.
[Image courtesy of S. Zhong and J. Giovannoni]

Breeding the tomatoes to contain the “uniform ripening” mutation disables GLK2, however. This change has the unintended effect of impairing the development of chloroplasts, the structures in plant cells that enable plants to photosynthesize. Impairing their development decreases the production of key ingredients that give tomatoes their sweetness.

These findings appear in the 29 June issue of Science.

The researchers suggest that manipulating GLK levels or expression patterns could help enhance the production and quality of tomatoes and other crops.

“This information about the gene responsible for the trait in wild and traditional varieties provides a strategy to recapture quality characteristics that had been unknowingly bred out of modern cultivated tomatoes,” said Powell.

“Now that we know that some of the qualities that people value in heirloom tomatoes can be made available in other types of tomatoes,” she said, “farmers can have access to more varieties of tomatoes that produce well and also have desirable color and flavor traits.”